The invention concerns a method of cell alignment and identification and calibration of robot tools. More particularly, the object of the invention is to determine and calibrate the operating point of a tool and the location of the robot relative to the work pieces for the purpose of ensuring the ability of the installed application to exactly repeat programmed paths of travel over long periods without re-programming being needed.
The state of the art is described in U.S Pat. No. 5 457 367. Methods of the kind defined in this patent specification have been used successfully for many years and have satisfied the most basic needs in a highly excellent manner.
However, the degree of absolute accuracy has proved insufficient to meet strict demands for accuracy in connection with simulation of geometry and off-line programming of robots or manipulators. Furthermore, it has been found that various types of defects, such as play that may arise in a tool or a robot axle, or friction in an axle, affect the method in a manner that may cause undetectable erroneous calibration. As a result, it may be necessary to update the program of movement of the robot by manually adjusting all programmed points. Not only is this a time-consuming task but in addition it results in the robot program being updated with erroneous calibration data. The consequence is loss of control of stop times in case of future incidents. Also, one has found that minor but commonly occurring non-ideal situations may give rise to poor calibration repeatability.
Another technique is shown in SE 508 161, according to which a spherical calibration tool is mounted on a robot and is moved several times past a calibration beam. By repeating the process a large number of times, using different robot configurations, calibration of the robot is made possible.
This method is, however, time-consuming and expensive because of the special spherical calibration tool used, which must be applied to the robot at each and every instance of calibration. In addition, a large number of cycles are required to calibrate a robot (for example 38 cycles in the case of a robot having six degrees of freedom). Furthermore, calibration of the kind described in SE 508 161 is not particularly useful in connection with a robot located in a work environment but this prior-art method is more particularly concerned with pre-calibration of a robot before installation thereof in the workplace. In addition, knowledge of the entire robot""s kinematic model is essential.
Two needs particularly are not met by the above method, viz.:
Cell calibration for the purpose of determining the position of the robot relative to the program of movement/workpiece with respect to which the robot movements are to be executed (so called cell alignment).
Identification and calibration of a robot tool. Both needs must be met to ensure the application-related function of a robot.
The object of the invention is to satisfy the need for a method of cell alignment and identification and calibration of robot tools that is sturdy, permanently installed, and absolutely accurate, that may be repeated with precision, and is automatic, quick, and inexpensive, in order to ensure that the robot is capable of repeating with precision programmed paths of movement over long periods of time while being governed by the originally programmed points or paths. In some cases, calibration is not possible, and then the identification may be used to automatically transform all programmed points in a manner corresponding to the change of the detected parameters.
This object of the invention is achieved by means of a method defined in the appended claims.
The method according to the invention offers considerable advantages. It is a high-technological and mathematically advanced method, which is applicable in the field, which directly operates on the objects of the application, and which optimizes the accuracy that is useful to the user.
It is not dependant on any particular calibration tool, but commonplace tools, such as welding electrodes or existing reference objects on gripping devices, work well to achieve the task.
The entire chain of precision of the application, including the position of the robot, the accuracy of the robot and the dimensions of the tool, is checked and/or is calibrated by analysis, resulting in information to the user of the accuracy available to him in the application executed by the robot. The method provides the user with a degree of accuracy that is optimized in one and the same operation and in terms of accuracy of use in the application in question. This method may give the application a higher degree of accuracy than the use of an individually calibrated robot comprising a separately and carefully measured tool and involving precisionmeasurement of the location of the robot relative to the workpiece, since this involves three separately suboptimized calibrations.
In addition, the method is applicable without knowledge of the kinematic model (equations) of the robot in question being necessary, a feature which broadens the practical applicability and the industrial usefulness, both because of the lesser requirements on separate adaptation to individual systems and because the kinematic models are difficult to access and in addition tend to become increasingly complex as the requirements from the market on absolute accuracy are becoming stricter (for which reason producing them is becoming increasingly time consuming unless one is in possession of special knowledge of the robot system in question).